1. Technical Field
The present invention relates to apparatus and methods for sensing fluid flow within a pipe using ultrasonic sensors in general, and to apparatus for improving signal-to-noise ratio for such sensors in particular.
2. Background Information
Normal incidence ultrasonic cross-correlation flow meters typically use pairs of ultrasonic sensors to detect modulation of ultrasonic signals transmitted through a moving media (e.g., fluid flowing within a pipe). Each pair of ultrasonic sensors includes a transmitter and a receiver. Coherent flow structures traveling within the flow cause modulation of the signals traveling through the flow. The modulation of the signals is measured at multiple axial locations and typically with multiple pairs of sensors. A signal analysis algorithm (e.g., a cross-correlation algorithm) is applied to determine the speed at which the coherent disturbances flow past the array of sensors.
Cross-correlation ultrasonic flow meters are well suited for clamp-on applications. One of the key challenges for a clamp-on ultrasonic flow meter is getting a sufficient amount of ultrasonic signal from the transmitter, through the first wall of the pipe, through the fluid passing within the pipe, through the second wall of the pipe, and to the receiver. The signal that follows the aforesaid path is typically referred to as “the fluid borne signal”. The fluid borne signal is only one component of the transmitted signal that arrives at the receiver. Another, often much larger, component of the transmitted signal, is the “structural borne signal”, or the “ring-around” signal. The ring-around signal travels within the pipe material and does not traverse the fluid passing within the pipe.
The ratio of the fluid borne signal component (considered the “signal” of interest for a flow meter) to the structural borne signal component (considered “noise” for a flow meter) of the arrived signal is a measure of the signal-to-noise for a flow meter application. In general, increasing the magnitude of the fluid borne signal component relative to the structural borne signal component (i.e., improving the signal-to-noise ratio) improves the operability and performance of ultrasonic flow meters.